Modern public switched telephone networks (PSTN) have separate paths for voice circuits (or other customer-utilized communication circuits) and for information transmitted throughout the network that controls the connection and disconnection of the voice circuits. By contrast, earlier versions of public switched telephone networks transmitted tone signals over the same circuits used as voice paths to control set-up and takedown of calls, to create billing records, and to provide other control for the telephone network.
In the late 1970s and early 1980s, American Telephone & Telegraph Company (AT&T) developed early versions of common channel interoffice signaling (CCIS). CCIS is essentially a network architecture for a switched telephone network in which information about a telephone call is transmitted over high speed data links that are separate from the voice circuits that are used to transmit the signals of the call itself. Early in the development of common channel interoffice signaling, it was recognized that the interoffice data signaling links could be designed to provide high speed digital data that could first determine whether a call could be completed prior to assigning trunk capacity to set up the voice link. Thus, with common channel interoffice signaling the identity of the called number can be transmitted over the interoffice signaling data links from the originating central office to the terminating central office.
The terminating central office is the central office that services the called number. If the called number is busy, data representing this information is transmitted back over the interoffice signaling link to the originating central office that locally provides an audible busy signal to the caller. Therefore, no long distance trunk capacity is occupied during this process, and the voice circuits remain free for other uses. If the called number is not busy, various devices in the network respond to the information about this call to assign interoffice trunks to set up a connection for the call. While the call is being set up, the originating central office, based on a signal from the terminating central office, returns an audible ring back tone to the caller. Once the called number is answered, an answer signal is passed from the terminating central office to the originating central office, the ring back tone is terminated, and the call is completed.
The public switched telephone network which evolved in the 1980s was designed by AT&T and includes the concept of the “Intelligent Network.” The intelligent network concept was expanded through industry consensus in the early 1990s and has become the Advanced Intelligent Network (AIN). The AIN enables telecommunications call control and database access from any computer or switching system connected to the Signaling System 7 (SS7) network. The Signaling System 7 (SS7) network refers to the current implementation of the CCIS control network used in the United States. The Advanced Intelligent Network (AIN) is a standard call control protocol which uses the SS7 network for message transport.
The AIN makes possible a variety of services for customers or subscribers to the AIN. One such service, referred to as call forwarding, permits a subscriber at a first subscriber location to have the phone calls that are destined for that first location forwarded to a second subscriber location on the telephone network. Therefore, the subscriber can have important phone calls forwarded so that these phone calls are not missed. Often, customers have multiple phones or subscriptions on the network such as one or more business phones, residential phones, and mobile or cellular phones, each having a separate phone number, and call forwarding allows a customer to forward business numbers to a cell phone while the customer is away from the office at lunch, meetings, or social functions. Even though several different phone numbers representing, for example, different businesses are forwarded to a single number, all forwarded calls will produce identical rings at the phone to which they have been forwarded. Therefore, the customer can not differentiate between a social call and important business calls. In addition, if one customer's calls are forwarded to another customer's phone, both customers need to be able to differentiate among the calls intended for either customer. In the case where the phone to which the calls have been forwarded is in use and that phone has a call waiting feature, the customer needs to know which announced waiting calls are potential important business calls.
One potential solution is a service known as caller identification where the identity of the calling party is displayed at the receiving phone. However, not all phones are equipped with this feature and this feature can be blocked by the calling party. In addition, this feature requires the user to be close to the phone and to view a display screen on the phone. These requirements present difficulties particularly for sight impaired subscribers. In addition, although the party placing the call may be displayed by caller identification, the number being called may not. Therefore, the customer viewing the caller identification screen must determine which number from among the multiple forwarded phone numbers is being called.